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Earnings Per Share - Additional Information (Detail) - shares
shares in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount (in shares) 31 2,600 55 400
Contingent Class A Common Stock | BGC Holdings Partnership Unit        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)     31,400 26,700
Contingent Class A Common Stock | Limited Partnership Unit        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)     31,400 26,700
Contingent Class A Common Stock | N Units        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)     31,400 26,700
Restricted Stock Units | Contingent Class A Common Stock        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)     31,400 26,700